This invention relates to packet data switching systems, and relates in particular to a means for preventing loss of service resulting from damage to an end user's access connection. It is of particular application to systems of the type known as “Ethernet” or those according to the IEEE 802.3 standard. In this specification, the term “Ethernet” embraces any network using Ethernet or IEEE 802.3 frames, including MPLS (Multiprotocol Label Switching) networks using Ethernet frames, and other networks operating on similar principles.
A simple Ethernet packet data switching system is shown at FIG. 1. For clarity only two access points are shown. These access points comprise terminal equipment 10, 11 which are typically owned by the customer and located on his premises, and provide the interface between the customer equipment and the public network. These access points are each connected by access connections 102, 113, to respective local nodes 12, 13. These local nodes direct the packets, by way of intermediate nodes e.g. 16 according to the address of the destination node carried in the header. On arrival at the destination node (e.g. 13) the packet is then forwarded to the access point 11. Note that any of the nodes 12-18 can serve as a local node for an associated access point. The individual connections 102, 126, 163, 113, etc may be of any type, such as wireless, wired or optical fibre. In the embodiment to be described the access connections are optical, but this is not to be taken as limitative.
Each local node 13 is typically connected to a plurality of user access points 11, 11a. As shown in United States Patent Application US2005/0047332, and in FIG. 2 of the attached drawings to the present specification, a passive optical coupler 132 is used to provide connections 111, 113 from a single local node 13 to several access points 11, 11a, with a multiplex imposed to separate data originating from, or destined for, each individual user. Passive couplers are well-known in the art: their function is to split a signal, or to combine two signals such that a single port can be connected to two ports. As will be described in relation to FIG. 7, passive couplers of this kind are also commonly used to connect separate input/output ports 31, 32 to a bidirectional port 30.
It will be seen that there is redundancy in the network, allowing packets to be transmitted by a variety of routes between the local nodes 12, 13. Thus, should an individual link 126 be overloaded or severed, data can still pass between the local nodes 12, 13 using some alternative route, e.g. via links 127, 178, 183.
In general, routing is carried out autonomously by the individual nodes 12-18, reading the address data in the packets. An overall control function 19 supervises the nodes (as indicated by the dotted lines) to monitor for link failures, and the like.
A problem arises if the link 102 between an access point 10 and its local node 12 is damaged. As there is no redundancy on this link 102, any damage to this connection severs the end-to-end link between the end users at the access points 10, 11. This connection 102, between the user's own terminal equipment at the access point 10 and the network access node 12, is necessarily partially located on the user premises, making it not only particularly vulnerable to damage, but harder for a network operator to gain access for repairs. A cost effective way of extending network redundancy into the access connections would therefore be desirable, especially if it can be achieved without modifying the equipment located on the user premises at the access points 10, 11, which may be owned by the customer of the provider of the respective access links 102, 113.
It is known, for example from United States Patent Application US2005/0047332 mentioned above, and as illustrated in FIG. 3 of the drawings attached to the present specification, to provide duplicate connections 112; 113 from a node 13 to each user access point 11. However, this system requires the user equipment to set up a main and a standby link to the local node, each link having its own physical termination point in the user equipment, and its own network address. According to the invention, this complexity can be avoided by providing an access connection for providing access between a user access point and an access node forming part of an Ethernet network (as hereinbefore defined) comprising a plurality of nodes, characterised in that the access connection comprises one or more passive couplers, by means of which the user access point is provided with a plurality of access connections to one or more access nodes such that data can be exchanged, over each access connection, between the user access point and the one or more access nodes, and means for controlling the access node or nodes such that only one of the access connections transmits the data received from the network to the user access point. It will be noted that, unlike the configuration used in the prior art reference, the present invention does not use the passive coupler to connect a single network access node to a plurality of user access points. Instead, the coupler is reversed such that it connects a single user access point to a plurality of network access nodes. This invention therefore requires minimal modification to the user terminal equipment to provide the security of a redundant path. Instead, the network access nodes are controlled to avoid collision in the data between the duplicate paths. Thus, the customer equipment is kept as simple as possible, and any complexity is in the operator's network, where it is easier to supervise and maintain.
Encapsulation of packets is common in some packet systems but not normally required in Ethernet systems. However, in this invention encapsulation is desirable in order to prevent the formation of loops within the network. In a preferred embodiment, it is therefore desirable for each access node to provide means for generating a distinct encapsulation of the data transmitted to it from the user access point for transmission over the network, and means for de-encapsulation of data destined for the user access point that it receives from elsewhere in the network, for transmission to the user access point.
The provision of a distinct encapsulation for each instance of data prevents the formation of loops within the network. At the destination access node or nodes, the data is de-encapsulated, resulting in two identical data streams (save for any loss or corruption en route). However, path and phase differences would cause multiple sets of de-encapsulated data to interfere with each other at the user equipment if both streams were to be successfully delivered to the destination user access points, through intact access connections Thus co-operation between the access paths is required to ensure that only one set of de-encapsulated data arrives at the user access point. This may be achieved by comparing de-encapsulated packets, and not forwarding any that have already been sent. However, it is preferred to designate a first access connection path as the path over which data is sent by default, monitoring the return connection path, and in the event that a loss of data is detected on that return path, switching to the other access connection path. The loss of access path due to fibre break is detected at the access node as a ‘Loss of Signal’ condition.
In a preferred arrangement, single-fibre links are used, typically implemented using ‘Bi-Di’ (Bi-directional transmission on a single fibre) SFP (Small Form Pluggable) optical modules. The use of two separate fibre connections (for transmit and receive) is possible, but in that case a Backward Defect Indication signal would need to be in place for the access node to detect a single-fibre break that causes a Loss of Signal condition on the user port.
The invention also provides an access node for an Ethernet network comprising means for receiving data packets over an access connection from a user access point, means for forwarding the packets to other nodes in a network, means for receiving data packets from other nodes in the network, and means for forwarding the packets over the access connection to a user access point, characterised by means for detecting traffic received over the access connection, means for comparing the detected traffic information relating to plurality of such connections to the same user access point, and means for selectively suspending the transmission of packets over the access connection to the user access point in accordance with that information.
The invention also provides an Ethernet network comprising a plurality of interconnected access nodes connected by access connections to respective user access points, wherein each access node provides means for receiving data transmitted to it from the respective user access point for transmission over the network, and means for transmitting data destined for the user access point that it receives from elsewhere in the network, the system being characterised in that at least one user access point is connected by way of a passive coupler to one or more access nodes by a plurality of access connections such that data can be exchanged between the user access point and the network by a plurality of routes using different access connections, and means for controlling the access nodes such that only one of the access connections transmits data received from the network to the respective user access point.
The invention allows Ethernet access links to customer sited equipment to be protected without the need for that equipment to support any protection scheme.
This increases the range of equipment that can be used for protected access links, as no hardware changes are necessary on the customer equipment other than the addition of couplers between the customer equipment and the access fibres. Small Form Factor Pluggable (SFP)-based interfaces (and Bi-directional SFP modules) may be used, removing the need to provide additional signalling for Remote Defect Indications.
The invention also provides a method of operating an Ethernet network such that data is transmitted to and from user access points by way of respective access nodes connected by access connections to the user access points, characterised in that a user access point is connected by way of a passive coupler to a plurality of access connections such that data can be exchanged between the user access point and the network by a plurality of routes using different access connections, and the access nodes are controlled such that only one of the access connections transmits data received from the network to the respective user access point.
The access nodes may be “virtually” distinct, but physically co-located and sharing some components: all that is required is that they have separate access connections to the user access point, and encapsulate and de-encapsulate the data separately.
The streams can be transported across the network together (with different encapsulations to prevent the formation of loops). One of the streams is designated as the active stream, and the or each other stream as standby streams. Standby streams are blocked, either at the destination or an intermediate node. A loss of signal condition on the access port for the currently active stream would signal the removal of the block on standby stream, allowing it to become the active stream. The original active stream would be redesignated as a standby stream, to avoid interference should it reinstate itself without intervention.
The paths between the access nodes may also have elements in common.
Multiple redundancy may be introduced by having more than one passive coupler: in this case all the access nodes must co-operate such that only one is transmitting to the user access equipment at any given time.